See also

Neurotransmitters are chemicals that are used to
relay, amplify and modulate signals between a neuron and another cell.
According to the prevailing beliefs of the 1960s, a chemical can be
classified as a neurotransmitter if it meets the following
conditions:

Excitatory and inhibitory

Release of excitory
neurotransmitters from the presynaptic membrane opens channels in
the postsynaptic membrane and leads to an increase in the concentration of sodium ions
within the postsynaptic cell and a decrease in that of potassium ions. This leads to
a 'depolarisation' of the
postsynaptic cell, which is propagated further along the cell
membrane by an action
potential.

Inhibitory neurotrasmitters encourage the
hyperpolarisation of the postsynaptic cell, making it less
likely to generate an action potential.

Whether a neurotransmitter acts in an excitatory
or inhibitory manner is determined by the reaction of the receptor
to its binding. Thus a given chemical can be excitatory at some
receptors and inhibitory at others.

NB: Neurotransmitters may be specific to
particular target organs and have multiple roles around the body.
For instance, Acetylcholine is released from both sympathetic and parasympathetic neurons.
ACH can be either excitatory to skeletal muscle cells or inhibitory
to both smooth muscle and cardiac muscle.

Neurotransmitter systems

Neurons expressing certain types
of neurotransmitters sometimes form distinct systems, where
activation of the system affects large volumes of the brain, called
volume transmission. The major neurotransmitter systems are the
noradrenaline
(norepinephrine) system, the dopamine system, the serotonin system and the
cholinergic
system.

Drugs targeting the neurotransmitter of such
systems affects the whole system; this fact explains the mode of
action of many drugs. Cocaine, for
example, blocks the reentering of dopamine back into the presynaptic neuron, leaving
these neurotransmitters in the synaptic gap longer. Since the
dopamine is in the synapse longer, the neurotransmitter rapidly hit
the receptors on the postsynaptic neuron cell,
and therefore causing happiness. Excess intake of cocaine can lead to physical
addiction. The physical addiction of cocaine is when the
neurotransmitters stay in the synapse so long , the the body
removes some receptor from the postsynaptic neuron. After
the drug affect is done, the person usually fells unhappy, because
now the neurotransmitters are less likely to hit the receptor since
the body removed many of them during the drug intake. Prozac is a
selective serotonin reuptake inhibitor (SSRI), hence
potentiating the effect of naturally released serotonin. AMPT prevents the
conversion of tyrosine to L-DOPA, the
precursor to dopamine; reserpine prevents dopamine
storage within vesicles; and deprenyl inhibits monoamine
oxidase (MAO)-B and thus increases dopamine levels.

Diseases may affect specific neurotransmitter
systems. For example, Parkinson's
disease is at least in part related to failure of dopaminergic
cells in deep-brain
nuclei, for example the substantia
nigra. Treatments potentiating the effect of dopamine
precursors have been proposed and effected, with moderate
success.

A brief comparison of the major neurotransmitter
systems follows:

Common neurotransmitters

Degradation and elimination

Neurotransmitter must be broken
down once it reaches the post-synaptic cell to prevent further
excitatory or inhibitory signal transduction. For example, acetylcholine, (ACH) (an
excitatory neurotransmitter), is broken down by acetylcholinesterase
(AchE). Choline is taken up
and recycled by the pre-synaptic neuron to synthesize more ACH.
Other neurotransmitters such as dopamine are able to diffuse away from their
targeted synaptic junctions and are eliminated from the body via
the kidneys, or destroyed in the liver. Each neurotransmitter has
very specific degradation pathways at regulatory points, which may
be the target of the body's own regulatory system or recreational
drugs.

=Sub-Neurotransmitters= In the process of
neurotransmission, certain unknown components are existent and
active, though their activities are subtle.

These components assist and desist procession of
neurotransmission by aiding communication or stimulating the
building blocks of the neurotransmitter itself. Examples are: